Cardiovascular disease, including atherosclerosis, is the leading cause of death in the United States. One method for treating atherosclerosis and other forms of arterial lumen narrowing is percutaneous transluminal angioplasty, commonly referred to as “angioplasty” or “PTA,” or “PTCA” when performed in the coronary arteries. The objective in angioplasty is to restore adequate blood flow through the affected artery, which may be accomplished by inflating a balloon of a balloon catheter within the narrowed lumen of the artery to dilate the vessel. Typically, inflation of the balloon is accomplished by supplying a pressurized fluid through an inflation lumen in the catheter which is connected to an inflation apparatus located outside of the patient's body. Similarly, applying suction to the inflation lumen collapses the balloon to its minimum dimension for initial placement of the balloon catheter within or removal from the target blood vessel.
A wide variety of angioplasty catheter designs and constructions are available. Typically, the catheter balloon is constructed of a thermoplastic elastomer (TPE) or other polymer configured to produce a nominal or labeled balloon diameter at a standard inflation pressure of six atmospheres. Because arteries differ in size, most balloon dilatation catheters are available in stepped dilation diameters ranging from approximately 1.5 millimeter to 4.0 millimeter in increments of 0.5 millimeter, or in the case of noncompliant balloons, in increments as small as 0.25 millimeter. After locating the stenosis in an artery or vessel utilizing a procedure such as an angiogram, the physician gauges the size of the affected vessel as accurately as possible and selects the corresponding balloon size to effectively open the lesion.
Dilatation balloons may be classified as being compliant, noncompliant or semi-compliant. Compliant angioplasty balloons are characterized by the balloon's ability to radially expand beyond its nominal diameter in response to increasing inflation pressure. Such balloons can be said to follow a stress-strain curve obtained by plotting balloon diameter versus inflation pressure. Noncompliant angioplasty balloons are characterized by a nearly flat stress-strain curve illustrating that the balloon diameter expands very little over the range of usable inflation pressures. It has been found that the optimal size of a dilatation balloon is about 0.9 to about 1.3 the size of the vessel being treated. See Nichols et al., Importance of Balloon Size in Coronary Angioplasty, J. American College of Cardiology, Vol. 13, 1094 (1989). If an undersized balloon is used, there is a high incidence of significant residual stenosis and a greater need for subsequent dilatation procedures. However, if an oversized balloon is used, there is an increased chance of coronary dissection. Therefore, physicians desire to use a balloon which will closely approximate the size of the occluded vessel or obstructed cavity being treated. Thus, when increased inflation pressures are required to open a resistant stenosis, physicians keep in mind that a compliant balloon will also be increasing in diameter. However, compliant balloons typically can suffer from some degree of plastic deformation during inflation such that, upon subsequent inflations, the balloon will achieve diameters greater than the diameters originally obtained at any given pressure.
Semi-compliant balloons have been developed that, as compared to compliant balloons, offer a reduced degree of radial expansion beyond their nominal diameter in response to increasing inflation pressure. Such balloons also resist plastic deformation during inflation such that the balloon diameter will follow a stress-strain curve even during repeated inflations. See U.S. Pat. No. 5,500,180. Thus, for several reasons, as a dilatation balloon expands in situ, it is desirable for the physician to accurately monitor the balloon diameter, but the only means available are by visual approximation of the balloon shown on a magnified x-ray image or by observing the pressure indicated on a balloon inflation device and trying to make a correlation between the indicated pressure and the actual balloon diameter.
In addition to balloon angioplasty procedures, stent prostheses are implanted within body lumens to provide artificial radial support to dilated, collapsing, weakened, and/or stenosed passageways, such as blood vessels of the body. Stent prostheses are typically constructed of a metal or polymer and are generally a hollow cylindrical shape. When a balloon-expandable stent is to be implanted, a balloon catheter carrying the stent mounted on its balloon is advanced to the target site, such as a stenosis. The balloon and accompanying stent are positioned at the location of the stenosis, and the balloon is inflated to radially expand and thereby implant the stent. As the balloon expands, it physically forces the stent to radially expand such that the outside surface of the stent comes into contact with the vessel wall. Thereafter, the balloon is deflated and the balloon catheter is withdrawn from the patient, leaving the stent in the expanded or deployed configuration. One criterion for successful stent deployment is apposition of the stent against the vessel wall, since any regions of the stent that protrude into the lumen may cause turbulent blood flow, which in turn may lead to acute thrombosis and arterial blockage. By “apposition” or “wall apposition” herein it is meant that at least the outer surface of the deployed stent is fully positioned against, i.e., makes contact with, the vessel wall. Proper stent apposition against the vessel wall is obtained by expanding the stent with a balloon inflated to the correct diameter.
Thus, it is desirable during both angioplasty and stent implantation procedures to accurately achieve the proper inflated diameter of the catheter balloon, and it is desirable for the physician to monitor the balloon diameter during balloon inflation to insure that the balloon is not under-inflated or over-inflated. Accordingly, there is a need for an apparatus and method that will effectively communicate real time balloon diameters to a physician during inflation of a catheter balloon. It is an object hereof to provide a system that is capable of determining and displaying an in vivo balloon diameter dimension during balloon inflation.